In most cases, Google Nest thermostats can charge their built-in battery using your system’s heating and cooling wires. But in a small number of situations you may need to connect a common or C wire to deliver enough consistent power to your thermostat.
A common wire is simply a wire that's dedicated to delivering constant power to your thermostat. Unlike the other wires connected to your thermostat it doesn't control heating or cooling functions.
Why Nest thermostats don't typically need a C wire
The Nest compatibility checker will tell you to connect a C wire
Why your Nest thermostat may need a C wire
In some cases, you may experience odd heating or cooling behaviors after installing your Nest thermostat. Connecting a C wire to your Nest thermostat may help resolve the following issues:
- Your system is making strange noises: chattering, stuttering, clicking or thumping - this can be caused by your system turning on and off rapidly
- Heating or cooling is always on, and won’t turn off
- Heating or cooling repeatedly turns on and off in a short period
- You see a Delayed or Starts in message on your Nest thermostat’s screen (for example: “Heating is delayed for 2:30 mins”)
- The system fan is always running or won’t turn on
- The system fan turns on and off repeatedly in a short period
The best solution for the above issues is to contact a local pro. They can diagnose and help fix any problems with your system and thermostat installation. If needed, they can also connect a common (C) wire or a C wire accessory, such as a Fast-Stat Common Maker, to your Nest thermostat.
See the following article for more information: